Method for manufacturing a semiconductor device

ABSTRACT

A process for fabricating a highly stable and reliable semiconductor, comprising: coating the surface of an amorphous silicon film with a solution containing a catalyst element capable of accelerating the crystallization of the amorphous silicon film, and heat treating the amorphous silicon film thereafter to crystallize the film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for manufacturing asemiconductor device having a crystalline semiconductor. The presentinvention further relates to an electro-optical device such as an activematrix liquid crystal device using the semiconductor device.

2. Prior Art

Thin film transistors (referred to simply hereinafter as “TFTs”) arewell known and are widely used in various types of integrated circuitsor an electro-optical device, and particularly used for switchingelements provided to each of pixels of an active matrix (-addressed)liquid crystal display device as well as in driver elements of theperipheral circuits thereof.

An amorphous silicon film can be utilized most readily as the thin filmsemiconductor for a TFT. However, the electric characteristics of theamorphous silicon film are disadvantageously poor. The use of a thinfilm of polysilicon (polycrystalline silicon), which is a crystallinesilicon, can solve the problem. Crystalline silicon is denoted as, forexample, polycrystalline silicon, polysilicon, and microcrystallinesilicon. The crystalline silicon film can be prepared by first formingan amorphous silicon film, and then heat treating the resulting film forcrystallization.

The heat treatment for the crystallization of the amorphous silicon filmrequires heating the film at a temperature of 600° C. or higher for aduration of 10 hours or longer. Such a heat treatment is detrimental fora glass substrate. For instance, a Corning 7059 glass commonly used forthe substrate of active matrix liquid crystal display devices has aglass distortion point of 593° C., and is therefore not suitable forlarge area substrates that are subjected to heating at a temperature of600° C. or higher.

According to the study of the present inventors, it was found that thecrystallization of an amorphous silicon film can be effected by heatingthe film at 550° C. for a duration of about 4 hours. This can beaccomplished by disposing a trace amount of nickel or palladium, orother elements such as lead, onto the surface of the amorphous siliconfilm.

The elements above (hereinafter referred to as “catalyst elementscapable of accelerating the crystallization of an amorphous siliconfilm” or simply as “catalyst elements”) can be introduced into thesurface of the amorphous silicon film by depositing the elements byplasma treatment or vapor deposition, or by incorporating the elementsby ion implantation. The plasma treatment more specifically comprisesadding the catalyst elements into the amorphous silicon film bygenerating a plasma in an atmosphere such as gaseous hydrogen ornitrogen using an electrode containing catalyst elements therein in aplasma CVD apparatus of a parallel plate type or positive columnar type.

However, the presence of the catalyst elements in a large quantity inthe semiconductor is not preferred, because the use of suchsemiconductors greatly impairs the reliability and the electricstability of the device in which the semiconductor is used.

That is, the catalyst elements are necessary in the crystallization ofthe amorphous silicon film, but are preferably not incorporated in thecrystallized silicon. These conflicting requirements can be accomplishedby selecting an element which tend to be inactive in crystalline siliconas the catalyst element, and by incorporating the catalyst element at aminimum amount possible for the crystallization of the film.Accordingly, the quantity of the catalyst element to be incorporated inthe film must be controlled with high precision.

The crystallization process using nickel or the like was studied indetail. The following findings were obtained as a result:

(1) In case of incorporating nickel by plasma treatment into anamorphous silicon film, nickel is found to intrude into the film to aconsiderable depth of the amorphous silicon film before subjecting thefilm to a heat treatment;

(2) The initial nucleation occurs from the surface from which nickel isincorporated; and

(3) When a nickel layer is deposited on the amorphous silicon film, thecrystallization of an amorphous silicon film occurs in the same manneras in the case of effecting plasma treatment.

In view of the foregoing, it is assumed that not all of the nickelintroduced by the plasma treatment functions to promote thecrystallization of silicon. That is, if a large amount of nickel isintroduced, there exists an excess amount of the nickel which does notfunction effectively. For this reason, the inventors consider that it isa point or face at which the nickel contacts the silicon that functionsto promote the crystallization of the silicon at lower temperatures.Further, it is assumed that the nickel has to be dispersed in thesilicon in the form of atoms. Namely, it is assumed that nickel needs tobe dispersed in the vicinity of a surface of an amorphous silicon filmin the form of atoms, and the concentration of the nickel should be assmall as possible but within a range which is sufficiently high topromote the low temperature crystallization.

A trace amount of nickel, i.e., a catalyst element capable ofaccelerating the crystallization of the amorphous silicon, can beincorporated in the vicinity of the surface of the amorphous siliconfilm by, for example, vapor deposition. However, vapor deposition isdisadvantageous concerning the controllability of the film, and istherefore not suitable for precisely controlling the amount of thecatalyst element to be incorporated in the amorphous silicon film.

SUMMARY OF THE INVENTION

In the light of the aforementioned circumstances, the present inventionaims to fabricate with high productivity, a thin film of crystallinesilicon semiconductor by a heat treatment at a relatively lowtemperature using a catalyst element, provided that the catalyst elementis incorporated by precisely controlling the quantity thereof.

In accordance with one aspect of the present invention, the foregoingobjects can be achieved by providing an amorphous silicon film with acatalytic element for promoting the crystallization thereof or acompound including the catalytic element in contact with the amorphoussilicon film, and heat treating the amorphous silicon with saidcatalytic element or said compound being in contact therewith, thereby,the silicon film is crystallized.

More specifically, a solution containing the catalytic element isprovided in contact with an amorphous silicon film in order to introducethe catalytic element into the amorphous silicon film.

It is another feature of the present invention to add a materialselected from the group consisting of Ni, Pd, Pt, Cu, Ag, Au, In, Sn,Pd, Sn, P, As and Sb into a silicon semiconductor film at a trace amountby contacting a solution containing said material with the silicon filmand then crystallize the silicon semiconductor film by heating at arelatively low temperature.

By utilizing the silicon film having a crystallinity thus formed, it ispossible to form an active region including therein at least oneelectric junction such as PN, PI or NI junction. Examples ofsemiconductor devices are thin film transistors (TFT), diodes, photosensor, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects and features of the present invention will bedescribed in detail with reference to the attached figures in which:

FIGS. 1A to 1D are cross sectional views for forming a crystallinesilicon film in accordance with the present invention;

FIGS. 2A and 2B are cross sectional views showing a formation of acrystalline silicon film in accordance with the present invention;

FIG. 3 is a graph showing a relation of a lateral growth length ofcrystals with respect to a concentration of nickel in a solution;

FIG. 4 is a graph showing a SIMS data with respect to nickel in asilicon region into which nickel is directly added;

FIG. 5 is a graph showing a SIMS data with respect to nickel in asilicon region where crystals grow along the lateral direction from theregion into which nickel is directly added;

FIGS. 6A to 6E show cross sectional views showing a manufacturingprocess of a semiconductor device in accordance with Example 3 of thepresent invention;

FIG. 7 shows a Ni concentration in a silicon film subjected to a plasmatreatment;

FIG. 8 is a Raman spectroscopic diagram with respect to a region intowhich nickel is directly added;

FIG. 9 is a Raman spectroscopic diagram with respect to a region wherecrystals grow in a lateral direction;

FIGS. 10A-10F are cross sectional views showing a manufacturing processof an electro-optical device in accordance with Example 4 of the presentinvention.

FIGS. 11A-11D are cross sectional views showing a manufacturing processof a TFT in accordance with Example 5 of the present invention;

FIG. 12 shows a schematic diagram of an active matrix typeelectro-optical device in accordance with Example 6 of the presentinvention;

FIGS. 13A and 13B are cross sectional views showing the formation of acrystalline silicon film in accordance with Example 7 of the presentinvention;

FIGS. 14A-14E are cross sectional views showing a manufacturing processof a TFT in accordance with Example 8 of the present invention; and

FIGS. 15A and 15B are schematic diagrams showing an arrangement of anactive layer of a TFT in accordance with Example 8 of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The use of a solution for adding nickel or the like according to thepresent invention is advantageous in the following points:

(a) The concentration of the catalyst element (e.g. nickel) in thesolution can be accurately controlled in advance;

(b) The amount of the catalyst element incorporated into the amorphoussilicon film can be determined by the concentration of the catalystelement in the solution so long as the surface of the amorphous siliconfilm is brought into contact with the solution; and

(c) The catalyst element can be incorporated at a minimum concentrationnecessary for the crystallization into the amorphous silicon film,because the catalyst element adsorbed by the surface of the amorphoussilicon film principally contributes to the crystallization of the film.

The word “including” or “containing” mentioned in the presentspecification may be understood as either (a) that the catalytic elementis simply dispersed in a solution or (b) that the catalytic element iscontained in a solution in a form of a compound. As a solution, variousaqueous solutions and organic solvent solutions can be used. Thosesolvents can be roughly classified into a polar solvent and a non-polarsolvent.

Water, alcohol, acid or ammonium can be used as a polar solvent.Examples of nickel compounds which are suitable for the polar solventare nickel bromide, nickel acetate, nickel oxalate, nickel carbonate,nickel chloride, nickel iodide, nickel nitrate, nickel sulfate, nickelformate, nickel acetyl acetonate, 4-cyclohexyl butyric acid, nickeloxide and nickel hydroxide.

Also, benzene, toluene, xylene, carbon tetrachloride, chloroform orether can be used as a non-polar solvent. Examples of nickel compoundssuitable for a non-polar solvent are nickel acetyl acetonate and 2-ethylhexanoic acid nickel.

Further, it is possible to add an interfacial active agent to a solutioncontaining a catalytic element. By doing so, the solution can be adheredto and adsorbed by a surface at a higher efficiency. The interfacialactive agent may be coated on the surface to be coated in advance ofcoating the solution.

Also, when using an elemental nickel (metal), it is necessary to use anacid to dissolve it.

In the foregoing examples, the nickel can be completely solved by thesolvent. However, even if the nickel is not completely solved, it ispossible to use a material such as an emulsion in which elemental nickelor nickel compound is dispersed uniformly in a dispersion medium.

When using a polar solvent such as water for dissolving nickel, it islikely that an amorphous silicon film repels such a solution. In such acase, a thin oxide film is preferably formed on the amorphous siliconfilm so that the solution can be provided thereon uniformly. Thethickness of the oxide film is preferably 100 Å or less. Also, it ispossible to add an interfacial active agent to the solution in order toincrease a wetting property.

Further, it is possible to conduct a rubbing treatment on the surface ofthe thin oxide film in order to give the surface an irregularity with auniform gap, width and direction. Such irregularity helps the solvent topermeate, thereby, increasing the uniformity of the size and directionsof crystal grains. Also, such a crystalline semiconductor film in whichcrystals are oriented in a particular direction is advantageous to beused for a semiconductor device in order to increase a uniformity ofdevice characteristics.

Also, when using a non-polar solvent such as toluene for obtaining asolution of 2-ethyl hexanoic acid nickel, the solution can be directlyformed on the surface of an amorphous silicon film. However, it ispossible to interpose between the amorphous silicon film and thesolution a material for increasing the adhesivity therebetween, forexample, OAP (containing hexamethyl disilazane as a main component,produced by Tokyo Oka Kogyo) which is used to increase adhesivity of aresist.

The concentration of the catalyst element in the solution depends on thekind of the solution, however, roughly speaking, the concentration ofthe catalyst element such as nickel by weight in the solution is 1 ppmto 200 ppm, and preferably, 1 ppm to 50 ppm, and more preferably 10 ppmor lower. The concentration is determined based on the nickelconcentration in the silicon film or the resistance against hydrofluoricacid of the film upon completion of the crystallization.

The crystal growth can be controlled by applying the solution containingthe catalyst element to a selected portion of the amorphous siliconfilm. In particular, the crystals can be grown in the silicon film byheating the silicon film in a direction approximately parallel with theplane of the silicon film from the region onto which the solution isdirectly applied toward the region onto which the solution is notapplied.

It is also confirmed that this lateral growth region contains thecatalyst element at a lower concentration. It is useful to utilize acrystalline silicon film as an active layer region for a semiconductordevice, however, in general, the concentration of the impurity in theactive region is preferably as low as possible. Accordingly, the use ofthe lateral growth region for the active layer region is useful indevice fabrication.

The use of nickel as the catalyst element is particularly effective inthe process according to the present invention. However, other usefulcatalyst elements include nickel (Ni), palladium (Pd), platinum (Pt),copper (Cu), silver (Ag), gold (Au), indium (In), tin (Sn), phosphorus(P), arsenic (As), and antimony (Sb). Otherwise, the catalyst elementmay be at least one selected from the elements belonging to the GroupVIII, IIIb, IVb, and Vb of the periodic table.

EXAMPLE 1

The present example refers to a process for fabricating a crystallinesilicon film on the surface of a glass substrate. Referring to FIGS.1A-1D, the process for incorporating a catalyst element (nickel in thiscase) into the amorphous silicon film is described below. A Corning 7059glass substrate 100 mm×100 mm in size is used.

An amorphous silicon film from 100 to 1,500 Å in thickness is depositedby plasma CVD or LPCVD. More specifically in this case, an amorphoussilicon film 12 is deposited at a thickness of 1,000 Å by plasma CVD(FIG. 1A).

Then, the amorphous silicon film is subjected to hydrofluoric acidtreatment to remove impurities and a natural oxide formed thereon, ifnecessary. This treatment is followed by the deposition of an oxide film13 on the amorphous silicon film to a thickness of from 10 to 50 Å. Anatural oxide film may be utilized as the oxide film. The precisethickness of the oxide film 13 is not available because the film isextremely thin. However, the natural oxide film is assumably about 20 Åin thickness. The oxide film 13 is deposited by irradiating anultraviolet (UV) radiation in an oxygen atmosphere for a duration of 5minutes. The oxide film 13 can be formed otherwise by thermal oxidation.Furthermore, the oxide film can be formed by a treatment using aqueoushydrogen peroxide.

The oxide film 13 is provided with an aim to fully spread the acetatesolution containing nickel, which is to be applied in the later step, onthe entire surface of the amorphous silicon film. More briefly, theoxide film 13 is provided for improving the wettability of the amorphoussilicon film. If the aqueous acetate solution were to be applieddirectly, for instance, the amorphous silicon film would repel theaqueous acetate solution to prevent nickel from being incorporateduniformly into the surface of the amorphous silicon film.

An aqueous acetate solution containing nickel added therein is preparedthereafter. More specifically, an aqueous acetate solution containingnickel at a concentration of 10 to 200 ppm, e.g. 100 ppm, is prepared.Two milliliters of the resulting acetate solution is dropped to thesurface of the oxide film 13 on the amorphous silicon film 12, and ismaintained as it is for a predetermined duration of time, preferably fora duration of 0.5 minutes or longer, e.g. for a duration of 5 minutes.Spin drying at 2,000 rpm using a spinner is effected for 60 secondsthereafter to remove the unnecessary solution (FIGS. 1C and 1D).

The concentration of nickel in the acetate solution is practically 1 ppmor more, preferably, 10 ppm or higher. The solution needs not be only anacetate solution, and other applicable solutions include those ofhydrochlorides, nitrates, and sulfates. Otherwise, those of organicoctylates and toluene can be used as well. In case of using the organicsolutions, the oxide film 13 need not be incorporated because thesolution can be directly applied to the amorphous silicon film tointroduce the catalyst elements into the film.

The coating of the solution is carried out at one time or may berepeated, thereby, it is possible to form a film containing nickel onthe surface of the amorphous silicon film 12 uniformly to a thickness ofseveral angstrom to several hundreds angstrom after the spin dry. Thenickel contained in this film will diffuse into the amorphous siliconfilm during a heating process carried out later and will function topromote the crystallization of the amorphous silicon film. By the way,it is the inventors' intention that the film containing nickel or othermaterials do not necessarily have to be in the form of a completelycontinuous film, that is, it may be discontinuous, for example, in theform of number of clusters.

The amorphous silicon film coated with one of the above solutions iskept as it is thereafter for a duration of 5 minutes. The finalconcentration of nickel catalyst element in the crystallized siliconfilm 12 can be controlled by changing this retention time, however, themost influencing factor in controlling the final concentration of nickelcatalyst element in the crystallized silicon film is the concentrationof the nickel catalyst element in the solution.

The silicon film coated with a nickel-containing solution thus obtainedis subjected to heat treatment at a temperature of 550° C. for aduration of 4 hours in a nitrogen atmosphere in a heating furnace. Thus,a thin film of crystalline silicon 12 is formed on the substrate 11.

The heat treatment can be effected at any temperature of 450° C. orhigher. If a low temperature is selected, however, the heat treatmentwould consume much time and result in a poor production efficiency. If aheat treatment temperature of 550° C. or higher were to be selected, onthe other hand, the problem of heat resistance of the glass substratemust be considered.

EXAMPLE 2

The present example refers to a process similar to that described inExample 1, except that a silicon oxide film 1,200 Å in thickness isprovided selectively to incorporate nickel into selected regions of theamorphous silicon film using the silicon oxide film as a mask.

Referring to FIGS. 2A to 2C, the process for fabricating a semiconductoraccording to the present example is described below. A silicon oxidefilm is deposited to a thickness of 1,000 Å or more e.g. 1200 Å as amask on an amorphous silicon film 12. The silicon oxide film 21,however, may be thinner than 1000 Å, e.g. 500 Å if the film issufficiently dense as a mask. The silicon oxide film 21 is patternedinto a predetermined pattern thereafter by means of a conventionalphotolithography technique. A thin silicon oxide film 20 is formed byirradiating a UV radiation in oxygen atmosphere for 5 minutes. Thethickness of the silicon oxide film 20 is presumably from about 20 to 50Å (FIG. 2A). The function of the silicon oxide film thus formed forimproving the wettability of the amorphous silicon film might beoccasionally provided by the hydrophilic nature of the silicon oxidefilm formed as the mask in case the solution is matched with the size ofthe mask pattern. However, this is a special case, and, in general, asilicon oxide film 20 is safely used.

Then, similar to the process described in Example 1, 5 milliliters (withrespect to a substrate 10 cm×10 cm in size) of an acetate solutioncontaining 100 ppm of nickel is dropped to the surface of the resultingstructure. A uniform aqueous film is formed on the entire surface of thesubstrate by effecting spin coating using a spinner at 50 rpm for aduration of 10 seconds. Then, after maintaining the state for a durationof 5 minutes, the resulting structure is subjected to spin drying usinga spinner at a rate of 2,000 rpm for a duration of 60 seconds. Duringthe retention time, the substrate may be rotated on the spinner at arate of 100 rpm or lower (FIG. 2B).

The amorphous silicon film 12 is crystallized thereafter by applyingheat treatment at 550° C. for a duration of 4 hours in gaseous nitrogen.It can be seen that the crystal growth proceeds along a lateraldirection from the region 22 into which nickel is introduced as shown byarrow 23 toward the region 25 into which nickel is not directlyintroduced.

In FIG. 2C, the reference numeral 24 shows a region in which the nickelis directly introduced to cause the crystallization and the referencenumeral 25 shows a region in which the crystallization proceedslaterally from the region 24.

It was confirmed through transmission electron microscopy (TEM) andelectron diffraction that:

(a) the crystals grown in a lateral direction are monocrystalline in theform of needle or column having uniform widths;

(b) the growth direction of the crystals are approximately parallel withthe substrate surface although it depends upon the film thickness; and

(c) the growth direction of the crystals are substantially aligned withthe [111] axis of the crystals.

From the foregoing facts, it can be concluded that the surface of thelateral growth region 25 has a plane which is perpendicular to the [111]direction, namely at least one of planes {111} and those expressed by{hkl} (h+k=1), for example, {110}, {123}, {134}, {235}, {145}, {156},{257}, or {167}, or the neighborhood thereof.

It should be noted that since crystalline silicon has a diamondstructure of which space group is indicated by Fd3m, when the aboveindex hkl is even-odd mixing, a forbidden reflection occurs and it cannot be observed with the electron diffraction.

FIG. 3 shows the relation between the distance (μm) of the crystalgrowth to the region 23 along the transverse (lateral) direction and thenickel concentration (ppm) in the aqueous acetate solution.

FIG. 3 reads that a crystal growth for a distance of 25 μm or longer canbe realized by preparing a solution containing nickel at a concentrationof 100 ppm or higher. It can be also assumed from FIG. 3 that a crystalgrowth along the lateral direction of about 10 μm can be obtained byusing an aqueous acetate solution containing nickel at a concentrationof 10 ppm.

The datum plotted in FIG. 3 are for the case that the structure was heldfor a duration of 5 minutes after applying the nickel-containing aqueousacetate solution. However, the distance of crystal growth along thelateral direction changes with the retention time.

In case of using an aqueous acetate solution containing nickel at aconcentration of 100 ppm, for instance, longer distance of crystalgrowth can be obtained with increasing retention time up to 1 minute.However, once a retention time of 1 minute or longer is set, the furtherincrease becomes insignificant.

In case an aqueous acetate solution containing nickel at a concentrationof 50 ppm is used, the retention time increases proportional to thedistance of the crystal growth along the lateral direction. However, theincrement tends to saturate with increasing retention time to 5 minutesor longer.

Furthermore, it should be noted that temperature greatly influences thetime necessary for a reaction to achieve an equilibrium. Accordingly,the retention time is also subject to the temperature, and a strictcontrol of the temperature is indispensable. Thus, the distance ofcrystal growth can be increased in total by elevating the temperature ofheat treatment and by elongating the duration of the heat treatment.

FIGS. 4 and 5 show the nickel concentration in a silicon film obtainedby introducing nickel using an aqueous acetate solution containing 100ppm nickel and thereafter heat treating the silicon film at 550° C. fora duration of 4 hours. The nickel concentration is obtained by secondaryion mass spectroscopy (SIMS).

FIG. 4 shows the nickel concentration of the region 24 shown in FIG. 2C,i.e., the region into which nickel is directly incorporated. FIG. 5shows the nickel concentration of the region 25 in FIG. 2C, i.e., theregion in which crystal growth occurred along the lateral direction fromthe region 22.

By comparing the data of FIG. 4 with that of FIG. 5, it can be seen thatthe nickel concentration of the region in which the crystal growthoccurs along the lateral direction is lower by about one digit ascompared with that of the region into which nickel is introduceddirectly.

It can be seen also that the nickel concentration in the crystallizedsilicon film in the region into which nickel is introduced directly canbe suppressed to a level of 10¹⁸ cm⁻³ by using an aqueous acetatesolution containing nickel at a concentration of 10 ppm.

Conclusively, it is understood that the nickel concentration in thecrystalline silicon region in which the crystal growth occurs along thelateral direction can be suppressed to 10¹⁷ cm⁻³ or lower by using anaqueous acetate solution containing nickel at a concentration of 10 ppmand effecting the heat treatment at 550° C. or higher for a duration of4 hours or longer.

In conclusion, it is possible to control the concentration of nickel inthe region 24 of the silicon film where the nickel is directly addedwithin a range of 1×10¹⁶ atoms/cm³ to 1×10¹⁹ atoms/cm³ by controllingthe density of the solution and the retention time and further tomaintain the concentration of the nickel in the lateral growth region 25below that.

For comparison, a sample is prepared through a process in which, insteadof using a nickel containing solution, an amorphous silicon film isexposed to a plasma which is produced by using an electrode containingan amount of nickel in order to add the nickel into the silicon (this iscalled as a plasma treatment), and further the silicon film iscrystallized by a heat annealing at 550° C. for 4 hours. The conditionof the plasma treatment is selected so that the same degree of a lateralcrystal growth can be obtained as in the case where an acetic acidcontaining nickel at 100 ppm is used. The SIMS data with respect to thissample is shown in FIG. 7. As can be seen, in the case of using a plasmatreatment, the nickel concentration in the lateral growth region ishigher than 5×10¹⁸ atoms/cm³ which is undesirably high for an activeregion of a semiconductor device. Accordingly, it is to be understoodthat the use of a solution is advantageous for minimizing theconcentration of the nickel in the lateral growth region.

FIG. 8 shows a result of Raman spectroscopy with respect to the regioncorresponding to FIG. 4, namely, the region where the nickel is directlyintroduced. FIG. 8 indicates that the crystallinity in this region isextremely high. Also, FIG. 9 shows a result of Raman spectroscopy withrespect to the region where the crystal grows laterally. As can be seen,even in the lateral growth area, the intensity of the Raman spectrum ismore than ⅓ of the intensity of the single crystal silicon. Accordingly,it is concluded that the crystallinity in the lateral growth region isalso high.

The crystalline silicon film thus fabricated by the process according tothe present invention is characterized in that it exhibits an excellentresistance against hydrofluoric acid. To the present inventors'knowledge, if the nickel is introduced by a plasma treatment, theresistivity of the crystallized silicon against a hydrofluoric acid ispoor. When it is necessary to pattern a silicon oxide film which isformed over the crystalline silicon film for forming a contact holetherethrough, a hydrofluoric acid is usually used as an etchant. If thecrystalline silicon film has a sufficiently high resistance against thehydrofluoric acid, a large selection ratio (the difference in theetching rate of the silicon oxide film and the crystalline silicon film)can be objected so as to remove the silicon oxide film alone.Accordingly, a crystalline silicon film having high resistance againstattack of hydrofluoric acid is of great advantage in the fabricationprocess of a semiconductor device.

EXAMPLE 3

The present example relates to a process for fabricating TFTs which areprovided to each of the pixels of an active matrix liquid crystaldisplay device, using a crystalline silicon film fabricated by theprocess according to the present invention. The TFTs thus obtained canbe applied not only to liquid crystal display devices, but also to awide field generally denoted as thin film integrated circuits (ICs).

Referring to FIGS. 6A to 6E, the process for fabricating a TFT accordingto the present example is described below. A silicon oxide film (notshown in the figure) is deposited to a thickness of 2,000 Å as a basecoating on a glass substrate. This silicon oxide film is provided toprevent the diffusion of impurities into the device structure from theglass substrate.

An amorphous silicon film is deposited thereafter to a thickness of1,000 Å in a manner similar to that used in Example 1. After removingthe natural oxide film by a treatment using hydrofluoric acid, a thinfilm of an oxide film is formed to a thickness of about 20 Å by means ofUV irradiation under a gaseous oxygen atmosphere.

The resulting amorphous silicon film having the oxide film thereon iscoated with an aqueous acetate solution containing nickel at aconcentration of 10 ppm. The resulting structure is retained for aduration of 5 minutes, and is subjected thereafter to spin drying usinga spinner. The silicon oxide film is removed thereafter using a buferredhydrofluoric acid, and a silicon film is crystallized by heating theresulting structure at 550° C. for a duration of 4 hours. The process upto this step is the same as that described in Example 1.

The silicon film thus crystallized is patterned to form an island-likeregion 104 as shown in FIG. 6A. The island-like region 104 provides theactive layer for the TFT. A silicon oxide film 105 is formed thereafterfor a thickness of from 200 to 1,500 Å at a thickness of 1,000 Å. Thesilicon oxide film functions as a gate insulating film (FIG. 6A).

The silicon oxide film 105 is deposited by means of RF plasma CVDprocess using TEOS (tetraethoxysilane). That is, TEOS is decomposed andthen deposited together with oxygen at a substrate temperature of 150 to600° C., preferably in the range of 300 to 450° C. TEOS and oxygen areintroduced at a pressure ratio of 1:1 to 1:3 under a total pressure of0.05 to 0.5 Torr, while applying an RF power of 100 to 250 W. Otherwise,the silicon oxide film can be fabricated by reduced pressure CVD ornormal pressure CVD using TEOS as the starting gas together with gaseousozone, while maintaining the substrate temperature in the range of from350 to 600° C., preferably, in the range of from 400 to 550° C. The filmthus deposited is annealed in oxygen or under ozone in the temperaturerange from 400 to 600° C. for a duration of from 30 to 60 minutes.

The crystallization of the silicon region 104 can be accelerated byirradiating a laser beam using a KrF excimer laser (operating at awavelength of 248 nm at a pulse width of 20 nsec) or an intense lightequivalent thereto. The application of RTA (rapid thermal annealing)using infrared radiation is particularly effective because the siliconfilm can be heated selectively without heating the glass substrate.Moreover, RTA is especially useful in the fabrication of insulated gatefield effect semiconductor devices because it decreases the interfacelevel between the silicon layer and the silicon oxide film.

Subsequently, an aluminum film is deposited to a thickness of from 2,000Å to 1 μm by electron beam vapor deposition, and is patterned to form agate electrode 106. The aluminum film may contain from 0.15 to 0.2 % byweight of scandium as a dopant. The substrate is then immersed into anethylene glycol solution controlled to a pH of about 7 and containing 1to 3% tartaric acid to effect anodic oxidation using platinum as thecathode and the aluminum gate electrode as the anode. The anodicoxidation is effected by first increasing the voltage to 220 V at aconstant rate, and then holding the voltage at 220 V for 1 hour tocomplete the oxidation. In case a constant current is applied as in thepresent case, the voltage is preferably increased at a rate of from 2 to5 V/minute. An anodic oxide 109 is formed at a thickness of from 1,500to 3,500 Å, more specifically, at a thickness of, for example, 2,000 Åin this manner (FIG. 6B).

Impurities (specifically in this case, phosphorus) are implanted intothe island-like silicon film of the TFT in a self-aligned manner by iondoping (plasma doping) using the gate electrode portion as a mask.Phosphine (PH₃) is used as a doping gas to implant phosphorus at a doseof from 1×10¹⁵ to 4×10¹⁵ cm^(−2.)

The crystallinity of the portion whose crystallinity is impaired by theintroduction of impurities is cured by irradiating a laser beam using aKrF excimer laser operating at a wavelength of 248 nm and a pulse widthof 20 nsec. The laser is operated at an energy density of from 150 to400 mJ/cm², preferably, in a range from 200 to 250 mJ/cm². Thus areformed N-type impurity regions (regions doped with phosphorus) 108. Thesheet resistance of the regions is found to be in the range of 200 to800 Ω/square.

This step of laser annealing can be replaced by an RTA process, i.e., arapid thermal annealing process using a flash lamp, which compriseselevating the temperature of the silicon film rapidly to a range of from1,000 to 1,200° C. (as measured on the silicon monitor). This method ofannealing is also called as RTP (rapid thermal process).

A silicon oxide film is deposited thereafter to a thickness of 3,000 Åas an interlayer dielectric 110 by means of plasma CVD using TEOStogether with oxygen, or by means of reduced pressure CVD or normalpressure CVD using TEOS together with ozone. The substrate temperatureis maintained in the range of 250 to 450° C., for instance, at 350° C. Asmooth surface is obtained thereafter by mechanically polishing theresulting silicon oxide film. An ITO coating is deposited thereon bysputtering, and is patterned to provide a pixel electrode 111 (FIG. 6D).

The interlayer dielectric 110 is etched to form contact holes in thesource/drain as shown in FIG. 6E, and interconnections 112 and 113 areformed using chromium or titanium nitride to connect the interconnection113 to the pixel electrode 111.

In the process according to the present invention, nickel isincorporated into the silicon film by using an aqueous solutioncontaining nickel at such a low concentration of 10 ppm. Accordingly, asilicon film having a high resistance against hydrofluoric acid can berealized and contact holes can be formed stably and with highreproducibility.

A complete TFT can be formed by finally annealing the silicon film inhydrogen in a temperature range of 300 to 400° C. for a duration of from0.1 to 2 hours to accomplish the hydrogenation of the silicon film. Aplurality of TFTs similar to the one described hereinbefore arefabricated simultaneously, and are arranged in a matrix to form anactive matrix liquid crystal display device.

In accordance with the present example, the concentration of the nickelcontained in the active layer is in the range of 5×10¹⁶ to 3×10¹⁸atoms/cm³.

As described above, the process according to the present examplecomprises crystallizing the portion into which nickel is introduced.However, the process can be modified as in Example 2. That is, nickelcan be incorporated to selected portions through a mask, and crystalsmay be allowed to grow from the portions in a lateral direction. Thisregion of crystal growth is used for the device. A device far morepreferred from the viewpoint of electric stability and reliability canbe realized by further lowering the nickel concentration of the activelayer region of the device.

EXAMPLE 4

This example is directed to a manufacture of a TFT used to control apixel of an active matrix. FIGS. 10A-10F are cross sectional views forexplaining the manufacture of the TFT in accordance with this example.

Referring to FIG. 10A, a substrate 201, for example glass substrate, iswashed and provided with a silicon oxide film 202 on its surface. Thesilicon oxide film 202 is formed through a plasma CVD with oxygen andtetraethoxysilane used as starting gases. The thickness of the film is2000 Å, for example. Then, an amorphous silicon film 203 of an intrinsictype having a thickness of 500-1500 Å, for example, 1000 Å is formed onthe silicon oxide film 202, following which a silicon oxide film 205 of500-2000 Å, for example 1000 Å is formed on the amorphous silicon filmsuccessively. Further, the silicon oxide film 205 is selectively etchedin order to form an opening 206 at which the amorphous silicon film isexposed.

Then, a nickel containing solution (an acetic acid salt solution here)is coated on the entire surface in the same manner as set forth inExample 2. The concentration of nickel in the acetic acid salt solutionis 100 ppm. The other conditions are the same as in Example 2. Thus, anickel containing film 207 is formed.

The amorphous silicon film 203 provided with the nickel containing filmin contact therewith is crystallized through a heat annealing at500-620° C. for 4 hours in a nitrogen atmosphere. The crystallizationstarts from the region under the opening 206 where the silicon filmdirectly contacts the nickel containing film and further proceeds in adirection parallel with the substrate. In the figure, a referencenumeral 204 indicates a portion of the silicon film where the siliconfilm is directly added with nickel and crystallized while a referencenumeral 203 indicates a portion where the crystal grows in a lateraldirection. The crystals grown in the lateral direction are about 25 μm.Also, the direction of the crystal growth is approximately along an axesof [111].

After the crystallization, the silicon oxide film 205 is removed. Atthis time, an oxide film formed on the silicon film in the opening 206is simultaneously removed. Further, the silicon film 204 is patterned bydry etching to form an active layer 208 in the form of an island asshown in FIG. 10B. It should be noted that the nickel is contained inthe silicon film at a higher concentration not only under the opening206 where the nickel is directly added but also at a portion where topends of the crystals exist. The patterning of the silicon film should bedone in such a manner that the patterned silicon film 208 should notinclude such portions at which nickel is contained at a higherconcentration.

The patterned active layer 208 is then exposed to an atmospherecontaining 100% aqueous vapor of 10 atm at 500-600° C., typically, 550°C. for one hour in order to oxidize the surface thereof and thus to forma silicon oxide film 209 of 1000 Å. After the oxidation, the substrateis maintained in an ammonium atmosphere (1 atm, 100%) at 400° C. At thiscondition, the silicon oxide film 209 is irradiated with an infraredlight having an intensity peak at a wavelength in the range of 0.6-4 μm,for example, 0.8-1.4 μm for 30-180 seconds in order to nitride thesilicon oxide film 209. HCl may be added to the atmosphere at 0.1 to10%. A halogen lamp is used as a light source of the infrared light. Theintensity of the IR light is controlled so that a temperature on thesurface of a monitoring single crystalline silicon wafer is set between900-1200° C. More specifically, the temperature is monitored by means ofa thermocouple embedded in a single crystal silicon wafer and istransferred back to the IR light source (feed back). In the presentexample, the temperature rising rate is kept constant in the range of50-200° C./sec. and also the substrate is cooled naturally at 20-100°C./sec. Since the IR light can heat the silicon film selectively, it ispossible to minimize the heating of the glass substrate.

Referring to FIG. 10C, an aluminum film is formed by sputtering methodto a thickness of 3000-8000 Å, for example, 6000 Å and then patternedinto a gate electrode 210. The aluminum film may preferably containscandium at 0.01-0.2%.

Referring to FIG. 10D, the surface of the aluminum electrode 210 isanodic oxidized to form an anodic oxide film 211 in an ethylene glycolsolution containing a tartaric acid at 1-5%. The thickness of the oxidefilm 211 is 2000 Å, which will determine the size of an offset gate areawhich is to be formed in a later step as discussed below.

Referring then to FIG. 10E, using the gate electrode and the surroundinganodic oxide film as a mask, an N-type conductivity impurity(phosphorous, here) is introduced into the active layer in aself-aligning manner by ion doping method (also called as plasma dopingmethod) in order to form impurity regions 212 and 213. Phosphine (PH₃)is used as a dopant gas. The acceleration voltage is 60-90 kV, forexample, 80 kV. The dose amount is 1×10¹⁵-8×10¹⁵ cm⁻², for example,4×10¹⁵ cm⁻². As can be seen in the drawing, the impurity regions 212 and213 are offset from the gate electrode by a distance “x”. Thisconfiguration is advantageous for reducing a leak current (off current)which occurs when applying a reverse bias voltage (i.e. a negativevoltage in the case of an NTFT) to the gate electrode. In particular,since it is desired that electric charges stored in a pixel electrode bemaintained without leaking in order to obtain an excellent display, theoffset configuration is particularly advantageous when the TFT is usedfor controlling a pixel of an active matrix as is the case in thepresent example.

Thereafter, an annealing is performed with a laser irradiation. As alaser, a KrF excimer laser (wavelength: 248 nm, pulse width: 20 nsec.)or other lasers may be used. The conditions of the laser irradiation inthe case of KrF excimer laser are: energy density is 200-400 mJ/cm², forexample, 250 mJ/cm², a number of shots is 2-10 shots per one site, forexample, 2 shots. Preferably, the substrate is heated to 200-450° C. toenhance the effect of the irradiation.

Referring to FIG. 10F, an interlayer insulating film 214 of siliconoxide is formed through a plasma CVD to a thickness of 6000 Å. Further,a transparent polyimide film 215 is formed by spin coating to obtain aleveled surface. Then, a transparent conductive film made of indium tinoxide for example is formed on the leveled surface by sputtering to athickness of 800 Å and patterned into a pixel electrode 216.

The interlayer insulating films 214 and 215 are provided with contactholes therethrough, through which electrode/wirings 217 and 218 canreach the impurity regions of the TFT. The electrode/wirings 217 and 218are formed of a metallic material, for example, a multi-layer oftitanium nitride and aluminum. Finally, an annealing in a hydrogenatmosphere of 1 atm is carried out at 350° C. for 30 minutes in order tocomplete a pixel circuit of an active matrix circuit having TFTs.

EXAMPLE 5

This example is directed to a manufacture of a TFT and will be describedwith reference to FIGS. 11A-1D. The same reference numerals will bereferred to for describing the same or similar elements as those of theprevious example.

Referring to FIG. 11A, a base film 202 of silicon oxide is initiallyformed on a Corning 7059 substrate 201 by sputtering to 2000 Å thick.The substrate is annealed at a temperature higher than a distortionpoint of the substrate following which the glass is cooled to atemperature less than the distortion point at a rate of 0.1-1.0°C./minute. Thereby, it is possible to reduce a contraction of thesubstrate due to a substrate heating which occurs later (for example,thermal oxidation, thermal annealing), as a result, a mask alignmentprocess will be facilitated. This step may be performed either before orafter the formation of the base film 201 or it may be done both beforeand after the formation of the base film 201. In the case of using theCorning 7059 substrate, the substrate may be heated at 620-660° C. for1-4 hours, following which it is cooled at 0.1-0.3° C. and taken outfrom a furnace when the temperature decreases to 400-500° C.

Then, an intrinsic (I-type) amorphous silicon film is formed to 500-1500Å thick, for example, 1000 Å through plasma CVD. The amorphous siliconfilm is crystallized in the same manner as in Example 1. Therefore, theredundant explanation will be omitted. After the crystallization, thesilicon film is patterned into an island form having a dimension of10-1000 microns square. Accordingly, a crystalline silicon film 208 inthe form of an island is formed as an active layer of a TFT as shown inFIG. 11A.

Referring to FIG. 11B, the surface of the silicon film is oxidized byexposing the surface to an oxidizing atmosphere to form an oxide film209. The oxidizing atmosphere contains an aqueous vapor at 70-90%. Thepressure and the temperature of the atmosphere is 1 atm and 500-750° C.,typically 600° C. The atmosphere is produced by a pyrogenic reactionfrom oxygen and hydrogen gases with a hydrogen/oxygen ratio being1.5-1.9. The silicon film is exposed to the thus formed atmosphere for3-5 hours. As a result, the oxide film 209 having a thickness of500-1500 Å, for example, 1000 Å is formed. Since the surface of thesilicon film is reduced (eaten) by 50 Å or more due to the oxidation, aneffect of a contamination of the upper most surface of the silicon filmdoes not extend to the silicon-silicon oxide interface. In other words,by the oxidation, it is possible to obtain a clean silicon-silicon oxideinterface. Also, since the thickness of the silicon oxide film is twotimes as the thickness of the portion of the silicon film to beoxidized, when the silicon film is originally 1000 Å thick and thesilicon oxide film obtained is 1000 Å, the thickness of the silicon filmremaining after the oxidation is 500 Å.

Generally, the thinner a silicon oxide film (gate insulating film) andan active layer are, the higher a mobility is and the smaller an offcurrent is. On the other hand, a preliminary crystallization of anamorphous silicon film is easier when its thickness is thicker.Accordingly, there was a contradiction in the crystallization processand electrical characteristics with respect to the thickness of theactive layer. The present example advantageously solves this problem.That is, the amorphous silicon film having a larger thickness isinitially formed so that a better crystalline silicon film can beobtained, following which the thickness of the silicon film is reducedby the oxidation, resulting in an improvement of characteristics of theactive layer of a TFT. Moreover, an amorphous component or grainboundaries contained in the crystalline silicon film tend to be oxidizedduring the thermal oxidation, resulting in a decrease in recombinationcenters contained the active layer.

After the formation of the silicon oxide film 209 through thermaloxidation, the substrate is annealed in a 100% monoxide dinitrogenatmosphere at 1 atm and 600° C. for 2 hours.

Referring to FIG. 11C, a silicon containing 0.01 to 0.2% phosphorous isdeposited through low pressure CVD to 3000-8000 Å thick, for example,6000 Å, and then patterned into a gate electrode 210. Further, using thegate electrode 210 as a mask, an N-type conductivity impurity is addedinto a portion of the active layer in a self-aligning manner by iondoping. Phosphine is used as a dopant gas. The doping condition issubstantially the same as in the Example 4. The dose amount is, forexample, 5×10¹⁵ cm⁻². Thus, N-type impurity regions 212 and 213 areformed.

Thereafter, an annealing is performed with a KrF excimer laser in thesame manner as set forth in Example 4. The laser annealing may bereplaced by a lamp annealing with a near infrared ray. The near infraredray is absorbed by crystalline silicon more effectively than byamorphous silicon. Accordingly, the annealing with the near infrared rayis comparable with a thermal annealing at 1000° C. or more. On the otherhand, it is possible to prevent the glass substrate from beingdetrimentally heated inasmuch as the near infrared ray is not soabsorbed by the glass substrate. That is, although a far infrared raycan be absorbed by a glass substrate, visible or near infrared ray ofwhich wavelength ranges from 0.5-4 μm are not so absorbed.

Referring to FIG. 11D, an interlayer insulating film 214 of siliconoxide is formed to 6000 Å thick by a plasma CVD. A polyimide may be usedinstead of silicon oxide. Further, contact holes are formed through theinsulating film. Electrode/wirings 217 and 218 are formed through thecontact holes by using a multilayer of titanium nitride and aluminumfilms. Finally, an annealing in a hydrogen atmosphere is conducted at350° C. and 1 atm for 30 minutes. Thus, a TFT is completed.

The mobility of the thus formed TFT is 110-150 cm²/Vs. The S value is0.2-0.5 V/digit. Also, in the case of forming a P-channel type TFT bydoping boron into source and drain regions, the mobility is 90-120 cm²/Vs and the S value is 0.4-0.6 V/digit. The mobility in accordance withthe present example can be increased by 20% or more and the S value canbe reduced by 20% or more as compared with a case where a gateinsulating film is formed by a known PVD or CVD.

Also, the reliability of the TFT in accordance with the present exampleis comparable to that of a TFT which is produced through a thermaloxidation at a temperature as high as 1000° C.

EXAMPLE 6

FIG. 12 shows an example of an active matrix type liquid crystal devicein accordance with the present example.

In the figure, reference numeral 61 shows a glass substrate, and 63shows a pixel area having a plurality of pixels in the form of a matrixeach of which is provided with a TFT as a switching element. Referencenumeral 62 shows peripheral driver region(s) in which driver TFTs areprovided in order to drive the TFTs of the pixel area. The pixels area63 and the driver region 62 are united on the same substrate 61.

The TFTs provided in the driver region 62 need to have a high mobilityin order to allow a large amount of electric currents to passtherethrough. Also the TFTs' provided in the pixel area 63 need to havea lower leak current property in order to increase a charge retentionability of pixel electrodes. For example, the TFTs manufactured inaccordance with Example 3 are suitable as the TFTs of the pixel area 63.

EXAMPLE 7

The present example is a modification of Example 1. That is, beforeforming a nickel acetate aqueous solution, a rubbing treatment isperformed on a silicon oxide surface in order to form number of minutescratches there.

Referring to FIG. 13A, a Corning 7059 substrate 11 having a siliconoxide film as a base film 18 is provided. The silicon oxide film isformed by sputtering to a thickness of 2000 Å for example. On thesilicon oxide film, an amorphous silicon film 12 is formed by plasma CVDto a thickness of 300-800 Å, for example, 500 Å. Subsequently, thesurface of the amorphous silicon film is treated with a hydrofluoricacid in order to remove a contamination or a natural oxide formedthereon. After that, a silicon oxide film of 10-100 Å thick is formed byexposing the substrate in an oxygen atmosphere with the surface beingirradiated with a UV light (not shown). The oxidation may be carried outwith a hydrogen peroxide treatment or thermal oxidation.

Then, fine scratches (unevenness or irregularity) are formed on thesilicon oxide film by a rubbing treatment as shown by reference numeral17. The rubbing treatment is carried out with a diamond paste. However,a cotton cloth or a rubber may be used instead of diamond paste. It isdesirable that scratches have a uniform direction, width and gap.

After the rubbing treatment, a film of nickel acetate is formed by spincoating in the same manner as in Example 1. The nickel acetate solutionis absorbed by the scratches uniformly.

Referring to FIG. 13B, the amorphous silicon film is then furnaceannealed at 550° C. for 4 hours in a nitrogen atmosphere like inExample 1. Thus, a crystalline silicon film is obtained. The grain sizesand orientation directions of the grains 19 in the thus obtained filmare more uniform than that obtained in Example 1. The grains 19 areextended in one direction and have an approximately rectangular orellipse shape or the like.

The dimension or number of scratches can be controlled by changing adensity of the diamond paste. Since it is difficult to observe thescratches with a microscope, the rubbing condition is determined in sucha manner that the size of grains or density of remaining amorphoussilicon in the obtained crystalline silicon film can be maximized. Inthis example, the condition of the treatment is selected so that lengthsof amorphous regions which remain after the crystallization be 1 μm orless, preferably, 0.3 μm or less.

In the case of Example 1 in which a rubbing treatment is not performed,there is a tendency that the nickel is not uniformly diffused andnon-crystallized regions in the form of 1-10 μm circles are observed.

Accordingly, the rubbing treatment improves the uniformity of theobtained crystals.

EXAMPLE 8

The present example is directed to a manufacturing process of TFTs forswitching pixels of an active matrix in accordance with Example 7. FIGS.14A-14E are cross sectional views showing the manufacturing process.

Referring to FIG. 14A, a silicon oxide film 202 is formed by a plasmaCVD to a thickness of 3000 Å on a substrate 201 made of Corning 7059glass (10 cm square). Then, an amorphous silicon film 203 is formed byplasma CVD to a thickness of 300-1000 Å, for example, 500 Å on thesilicon oxide film 202.

The thus formed amorphous silicon film is crystallized by the process asset forth in Example 7. After the thermal crystallization, a laserannealing is performed in order to improve the crystallinity with a Krexcimer laser (248 nm wavelength) having a power density 200-350 mJ/cm².As a result, amorphous components which remain in the crystallinesilicon film are completely crystallized.

After the crystallization, the silicon film 203 is patterned into anisland form silicon film 208 as shown in FIG. 14B. At this time, thelocation and the direction of the silicon island with respect to grainboundaries can be selected in such a manner as shown in FIGS. 15A and15B.

When an electric current of a TFT crosses grain boundaries, the grainboundaries function as a resistance. On the other hand, the electriccurrent is easy to flow along grain boundaries. Accordingly, theelectrical characteristics of a TFT is greatly influenced by the numberand direction of the grains (grain boundaries) included in the channelregion. For example, when there are a number of TFTs, a leak currentproperty of each TFT varies depending upon the number and direction ofthe grains contained in the channel region thereof.

The above problem becomes serious when the size of the grains isapproximately the same as the size of the channel or is smaller thanthat. When the channel is sufficiently larger than grains, thisdispersion is averaged and is not observed significantly.

For example, if there is no grain boundary in the channel, it can beexpected that the TFT has an electrical property which is the same asthat of a single crystalline TFT. On the other hand, when grainboundaries extend through the island along a direction of a draincurrent, the leak current becomes larger. In contrast, when grainboundaries extend in a direction perpendicular to a direction of a draincurrent, the leak current becomes smaller.

When TFTs are arranged in such a manner that its drain current flows ina direction along the rubbing direction, since crystals lengthen alongthe rubbing direction, the number of grain boundaries included in achannel tends to be nonuniform and therefore the leak current is likelyto disperse. Moreover, the intensity of the leak current becomes largerbecause the grain boundaries are aligned with the direction of the draincurrent as shown in FIG. 15A. On the other hand, as shown in FIG. 15B,if a drain current flows in a direction perpendicular to the rubbingdirection, the off current property can be stabilized. This is becausethe width of the grains 19 are approximately constant and the number ofgrains existing in the channel region 26 can be made constant. Inconclusion, it is desirable to arrange the active region 208 in such away that a drain current of a TFT flows in a direction perpendicular tothe direction of grain boundaries, i.e. the rubbing directions.Moreover, the rubbing treatment makes the size of crystal grainsuniform, which results in that non-crystallized region can beepitaxially crystallized by a subsequent laser irradiation.

As shown in FIG. 14B, a silicon oxide film of 200-1500 Å thick, forexample, 1000 Å thick is formed as a gate insulating film 209 throughplasma CVD.

Then, an aluminum containing Si at 1 weight % or Sc at 0.1 to 0.3 weight% is sputter formed to 1000 Å to 3 μm, for example 5000 Å, followingwhich it is patterned into a gate electrode 210. The aluminum electrodeis then subjected to an anodic oxidation process using an ethyleneglycol solution containing a tartaric acid at 1-3%. The pH of theelectrolyte is about 7. A platinum electrode is used as a cathode whilethe aluminum electrode is used as an anode. The voltage is increasedwith an electric current maintained constant until it reaches 220 V andthen this condition is maintained for one hour. As a result, an anodicoxide film 211 is formed to a thickness of 1500-3500 Å, for example 2000Å.

Referring to FIG. 14C, an impurity having one conductivity type (boron)is introduced into the silicon island through an ion doping method withthe gate electrode 210 used as a mask in a self-aligning manner.Diborane (B₂H₆) is used as a dopant gas. The dose amount is 4-10×10¹⁵cm⁻². The acceleration voltage is 65 kV. Thus, a pair of impurityregions (p-type) 212 and 213 are obtained.

Thereafter, the impurity regions 212 and 213 are activated byirradiating KrF excimer laser (248 nm wavelength, 20 nsec. pulse width).The energy density of the laser beam is 200-400 mJ/cm², preferably,250-300 mJ/cm².

Referring to FIG. 14D, an interlayer insulating film 214 made of siliconoxide is formed through plasma CVD to a thickness of 3000 Å. Then, acontact hole is formed on the impurity region 212 (source) through theinterlayer insulating film 214 and the gate insulating film 209 byetching. An aluminum film is then formed by sputtering and patterned toform a source electrode 217.

Referring to FIG. 14E, silicon nitride is deposited through plasma CVDto 2000-6000 Å as a passivation film 215. A contact hole is formed onthe impurity region (drain) 213 through the passivation film 215,interlayer insulating film 214 and the gate insulating film 209 byetching. Finally, an indium tin oxide film (ITO) is formed into a pixelelectrode 216. Thus, a pixel TFT is obtained.

While the present invention has been disclosed in preferred embodiments,it is to be understood that the scope of the present invention shouldnot be limited to the specific examples of the embodiments. Variousmodifications may be made.

For example, the nickel containing film may be formed by using anon-aqueous solution such as alcohol. When using an alcohol, thesolution may be directly formed on the amorphous silicon film withoutusing an oxide film. Specifically, a nickel containing compound such asnickel acetyl acetonate may be dissolved by ethanol. This material canbe decomposed during the heating for the crystallization because thedecomposition temperature thereof is relatively low. The amount of thenickel acetyl acetonate is selected so that the concentration of thenickel in the solution is controlled to be 100 ppm. The nickelcontaining film can be obtained by coating the solution and then driedby a spin dry method at 1500 rpm for 1 minute. Also, since the contactangle of the alcohol is smaller than that of water, the amount of thesolution used for forming the film may be smaller than in the case whena water solution is used. In this case, a drop of 2 ml with respect to100 mm square is appropriate. The subsequent steps for forming thecrystalline silicon may be entirely the same as those explained in thepreferred embodiments.

For another example, an elemental nickel may be dissolved by an acid.That is, a nitric acid of 0.1 mol/l is used as an acid. Nickel powder isdissolved in this acid at 50 ppm.

1. A thin film transistor comprising: a semiconductor film comprisingcrystalline silicon on an insulating surface; a channel regioncomprising at least one silicon crystal formed in the semiconductorfilm; source and drain regions in the semiconductor film with thechannel region therebetween; a gate electrode adjacent to the channelregion, wherein said silicon crystal has a [111] axis approximatelyparallel with said insulating surface.